This invention relates to water recovery and, in particular, to water recovery for use in high temperature fuel cell systems.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode electrode and a cathode electrode separated by an electrolyte, which serves to conduct electrically charged ions. High temperature fuel cells, such as molten carbonate fuel cells, operate by passing a reactant fuel gas through the anode, while oxidant gas comprising carbon dioxide and oxygen is passed through the cathode.
Reactant gases supplied to the fuel cell, and in particular, the reactant fuel gas supplied to the anode, must be sufficiently humidified to maintain a desired carbon to steam ratio in the fuel for proper and efficient fuel cell operation, to provide sufficient ionic conductivity of the reactant gases and to prevent carbon deposition in the fuel cell. The amount of water consumed by a fuel cell system for humidifying reactant gases is usually significant and requires a continuous supply of water to the fuel cell system.
The electrochemical reaction between the reactant fuel gas and the oxidant gas produces water vapor outputted as part of heated fuel cell exhaust gases including a heated cathode exhaust gas and a heated anode exhaust. In order to limit or eliminate water supplied to the fuel cell system from external sources, it is desired to separate water vapor in the fuel cell exhaust gases from other exhaust components and to recycle the separated water to humidify the reactant gases. An example of a water recycling system is disclosed in U.S. Pat. Nos. 5,068,159 and 4,039,579, which teach using a cooler and condenser to separate water from the anode exhaust stream, and thereafter passing the separated water through a boiler and a heater and feeding the water to the inlet of the anode compartment.
Another U.S. Pat. No. 7,060,382, assigned to the same assignee hereof, discloses a system in which a water transfer assembly in the form of a partial-pressure swing water transfer wheel is used to separate and transfer water vapor in anode exhaust as water to the fuel feed. This patent also discloses a system in which this transfer is carried out by using heat exchangers where the anode exhaust is cooled by the oxidant supply gas, water recycle vaporization and/or a cooling water or an air fan. After being cooled, the stream is fed to a scrubbing and blow-down assembly where the electrolyte contaminated water is removed. The resultant stream is then further cooled in a heat exchanger by an air fan or cooling water and the stream then fed to a condensing unit. At this unit, the water is removed and fed to the fuel feed, while the stream is further fed to a carbon dioxide transfer assembly.
Many conventional systems for separating water in the fuel cell exhaust employ complex and costly cooling systems for cooling of the hot anode exhaust gas leaving the fuel cell in order to condense a sufficient amount of water from the exhaust. Such cooling of the anode exhaust in conventional systems results in power consumption, increasing the operating costs of the system and complicates the pressure balance between the anode and the cathode. Typically, anode exhaust is not separately removed from the fuel cell compartment, and thus, implementing anode exhaust cooling requires special piping which cannot be easily added to an existing system. Cathode exhaust is normally vented to the atmosphere and water recovery from the cathode exhaust stream may be accomplished without impacting the typical operation of a fuel cell and can be easily retrofitted on existing units. It also works well with units which recover heat from the cathode exhaust since heat recovery assists in cooling the exhaust. Some systems transfer water from the anode exhaust since cathode exhaust gas requires greater cooling than the anode exhaust to transfer a sufficient amount of water due to the presence of spent oxidant gas in the cathode exhaust. In particular, the anode exhaust typically has to be cooled to about 140° F. to condense a sufficient amount of water, while the cathode exhaust has to be cooled to about 115° F. Nevertheless, because of the ease of installation, particularly when cathode exhaust is used in water recovery systems with waste heat recovery, water cathode exhaust water recovery systems are often preferred.
It is therefore an object of the present invention to provide an improved water transfer assembly capable of separating and transferring water from cathode exhaust gases without requiring significant power for operation and without requiring complex cooling equipment integrated into the fuel cell system.
It is another object of the present invention to provide a water transfer assembly which produces water free of electrolyte contamination, thus eliminating the need for a water treating system.
It is a further object of the present invention to provide a water transfer assembly which results in greater manufacturing and operating efficiencies in the fuel cell system.